Video tape recorder system having means for suppressing video track crossover noise during slow and fast motion operation

ABSTRACT

A helical scan video tape recorder suitable for slow and fast motion modes of operation is disclosed. A plurality of video fields are recorded in each video track extending across the video tape such that the burst of noise produced when the magnetic head crosses over the guard band between separate video tracks during the slow or fast mode of operation of the video recorder is encountered in only one of the field regions in the video track. This burst of noise is utilized to control the television monitor circuit so that the video field region which is free of noise is reproduced on the monitor while the video field region which contains the noise is blanked out from the monitor. As the noise progresses from one field region to the succeeding field region in the video track, means is provided for automatically switching to display of the noise-free field region. Means is provided for at times controlling the brightness of the monitor to compensate for those instances at switchover where two fields are reproduced in succession without the intervening blank period. A ramp generator circuit is provided for compensating for the change in timing of the vertical sync pulses during the slow and fast motion periods. In another embodiment, an additional magnetic head is utilized for reproducing the noise-free video field during the time period that the monitor would ordinarily be blanked out to thereby provide a 100 percent duty cycle in the video monitor.

BACKGROUND OF THE INVENTION

In the well known helical scan video tape recorders, the magnetic videotape has a plurality of video tracks recorded in an angularly disposeddirection across the tape, the individual video tracks being separatedby a guard band. Each individual video track records a complete field orframe of a television picture, the vertical sync signal of the fieldbeing recorded at the top and bottom ends of each video track. Aplurality of horizontal sync signals are also recorded in eachindividual track, for example, 262_(1/2) horizontal sync signals orlines per individual video track.

In the playback mode of operation, the magnetic tape passes in a helicalwrap around the rotating magnetic head of the recorder, the head tracinga path centered through one of the video tracks and reproducing thetelevision field recorded therein. As the recorder head passes off theone video track at the upper edge of the lower wrap of the tape, itpasses onto the next video track at the bottom edge of the upper wrap ofthe video tape and reproduces the field recorded therein. Thus, therecording head traces each individual video track from bottom to top insuccession. The speed of travel of the video tape and the speed of therotating recording head are so adjusted that the recording head is madeto be centered in the video track while automatically operated speedcontrol circuits are utilized to maintain the proper tracking betweenvideo track and recording head.

When the relative speed between the magnetic head and the video tape ischanged from the normal motion speed, for example where slow forward orreverse or fast motion reproduction speeds are adopted, the magnetichead no longer traces the center tracking of the video track but anglesoff on a different path, the actual direction being determined by theextent of the speed change as well as the direction of the change. Wherethe speed is decreased in the forward direction, the magnetic head willfollow a path bridging over the guard band and onto the forward adjacentvideo track. With an increased speed, the path of the magnetic headcrosses the guard band and moves onto the next rearward video track.

For each revolution of the magnetic head, one portion of the field inone track is reproduced, followed by a portion of the field from theadjacent track after the crossover. Because of the very closerelationship in time between the picture recorded on one track and thepicture on the adjacent track, it is difficult for the eye to detect achange in the monitor picture between the portion reproduced from onetrack and the following portion reproduced from the adjacent track.However, the magnetic head, in passing from one track to the other trackand over the guard band therebetween, will produce a short burst ofnoise which appears at the junction of the two picture portions as anarrow horizontal band or series of streaks across the televisionpicture.

Due to the fact that the magnetic head passes between the successivevideo tracks at a guard band crossover point which moves progressivelybetween the bottom and top edges of the tape, for slow motion forwardspeeds the noise band travels from the upper edge of the picture tubedown to the lower edge of the picture tube and then starts over again atthe top. This noise band appearing in the picture during slow motion andcontinuously moving down the screen is very disturbing to the eye as thepicture is being viewed. For fast motion speed and slow motion reverse,the disturbing noise band appears at the bottom of the picture and movesup.

BRIEF DESCRIPTION OF THE INVENTION

It is the object of the present invention to provide a video recordersystem in which the speed of motion of the picture on the monitor may bechanged from normal motion, for example, a change to slow motion or fastmotion, and wherein the picture disturbance produced by the passage ofthe magnetic head between adjacent video tracks and over the guard bandbetween the tracks may be eliminated from the video picture on themonitor.

In the present invention two separate fields are recorded on eachindividual video track from the video camera, the fields being recordedat a rate of 120 per second rather than the standard of 60 per second.The vertical sync pulse between the two separate fields in each track isrecorded at the midpoint of the track. Thus, for each revolution of themagnetic head, two separate fields are reproduced. When the speed of thevideo tape is changed for fast or slow motion, the video head will crossover the guard band between two adjacent tracks only once per revolutionand this crossover point will occur in the region between the fieldsrecorded in the upper half of the tape, depending upon the particularlocation of the crossover point at that particular instance in time.Although the crossover point between successive video tracks will changeits location along the guard band from top to bottom, it will occur atonly one particular place for each revolution of the head. Therefore,the noise burst produced by the head crossover between video tracks willoccur in one field or the other for any particular one video track butnot in both fields on the same revolution of the head. By selecting forreproduction on the monitor only that half of the video track which isfree from the noise burst produced by crossover, the noise will not bereproduced on the television screen.

Since a picture will be reproduced on the monitor only during one-halfof a revolution of the head and the picture will be blanked from thescreen during the other half of the period, there will be some reductionin the brightness of the picture. However, because of the nature of thehuman eye and the persistence of the picture tube, this reduction induty cycle is hardly discernible by the viewer. On the other hand, theelimination of the noise burst travelling across the picture greatlyimproves the viewability of the picture to the viewer.

Selection of the noise-free field to be shown on the monitor is madeautomatically under control of the crossover noise bursts. Noise pulsesoccurring in one field region on the tape will cause selection of theother field region for display. As the position of the noise pulsesmoves from the one field region to the other field region, the displayon the monitor switches from the display of said one field region todisplay of the other field region under control of the noise pulses.

At the point of switchover from reproduction of one field region toreproduction of the other field region, the last field ON time in oneseries will be followed immediately by an ON time for the first field inthe next series so that no blank on OFF time will occur between thesetwo reproduced fields. To the viewer of the monitor, the two successiveON field regions will appear as a momentary brightening of the pictureand will result in a flickering sensation to the viewer. To eliminatethis flickering, means is provided for lowering the brightness of thepicture on the monitor during the time period that these two successivefield regions are being reproduced on the monitor at the switchover.

In another embodiment of the invention, a second magnetic head and videoamplifier is utilized, the head being spaced 180° from the first videohead, the second head and associated circuitry operating to reproducethe noise-free field on the video monitor during the period of time thatthe first reproducer head is encountering the video field region withthe noise located therein, i.e. during what would normally be the blankfield period on the video monitor. A 100 percent duty cycle for thetelevision monitor is therefore obtained.

These and other features of the present invention will become apparentfrom a perusal of the following specification taken in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the tape transport arrangement of a typicalhelical scan video tape recorder which may be utilized in practicing thepresent invention.

FIG. 2 is a top view of the tape transport apparatus of FIG. 1.

FIG. 3 is a schematic representation of the prior art video tape showingthe arrangement of the slanted video traces and the paths therecord-reproduce head follows at various tape speeds.

FIG. 4 is a schematic representation of video tape utilized in thepresent invention and showing the new arrangement of the video tracks.

FIG. 5 illustrates the arrangement of the sheets of drawings of FIGS. 5Athrough 5D which show a schematic diagram of one embodiment of a videorecorder control circuitry employing the present invention.

FIG. 6 shows a plurality of electrical traces illustrating the operationof certain of the components shown in FIG. 5.

FIG. 7 is an enlarged view of one portion of a video track on themagnetic tape taken at section line 7--7 of FIG. 4, and

FIG. 8 is a block diagram of another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to FIGS. 1, 2, 3 and 5A, the helical scan video tapetransport of a typical video recorder 10 to which the present inventionrelates comprises a tape supply reel 11 and supply reel motor 12,take-up reel 13 and take-up reel motor 14, tape drive capstan 15 andcapstan drive motor 16, capstan pinch roller 17, tape guide posts 18,erase head 19, audio and control track head 21 and the helical scan drumassembly 22. The scan assembly 22 consists of a tape drum 23 mounted ata slight angle with respect to the vertical, the drum having a lowersegment 24 separated from an upper segment 25 by an annular gap 26sloped slighty relative to the horizontal. A head disk 27 is rotatablymounted within the drum on the drive shaft 28 driven by head drive motor29, a magnetic record-reproduce video head 31 being secured at one pointon the periphery of the disk 27 (See FIG. 5A).

The magnetic tape 32 is threaded past the erase, and audio and controltrack heads 19 and 21, respectively, and is wound in one complete loopin a helical manner about the tape drum 23. The upper and lower edges ofthe tape 32 are placed in abutting relationship at their crossover pointat the beginning and ending point of the tape loop. The tape is threadedpast a tape tensioning arm 33 and wound on the take-up reel 13. The tape32 is driven by means of capstan 15 and capstan pinch roller 17 from theleft-hand reel 11 to the right-hand reel 13 during record and playbackand the head disk 27 and magnetic video head 31 rotate in a clockwisedirection, the tape 32 and head 31 thus travelling in oppositiondirections. The tape 32 passes over the gap 26 in the drum 23 while thehead rotates rapidly around in the gap 26 and traces a plurality ofsuccessive, spaced-apart video tracks 34 on the tape 32 which are slopedat some selected angle determined by the physical characteristics of thetape drive, a typical slope being 4°45'. The video tracks 34 are about 6mil wide and are spaced apart by guard bands 35 about 3 mil wide. Duringthe record mode of operation the recorder system lays down the slopedvideo tracks 34, an audio track near and parallel to one edge of thetape 32, and a control track consisting of a plurality of spaced-apartcontrol pulses near and parallel to the other edge of the tape. Duringplayback, the speeds of the tape 32 and the head 31 are synchronized bymeans of the control track signals so that, for the normal speed mode ofoperation, the video head 31 is centered on the recorded video tracks 34in succession and follows the path indicated by the dotted line 36.

In a typical 60 field video recorder, the video head 31 rotates at aspeed of 3,600 revolutions per minute with the tape moving at alongitudinal speed of approximately 7 inches per second. Each rotationof the video head records (or plays back) one video field, including thevertical sync and blanking pulses 37 which are recorded at the top andbottom edge of the tape where the head crossover occurs, and theplurality of horizontal sync pulses 38.

Helical scan video recorders are well known in the art and are fullydescribed in published material and no further detailed description ofthe tape transport apparatus, the video and audio record and reproducecircuitry, or the television monitor circuitry will be given here exceptas the operation of specific portions relate to and are desirable for anunderstanding of the present invention. The present embodiment wasdeveloped using a Model IVC-800 color Video Recorder manufactured andsold by the International Video Corporation of Mountain View, Calif. anda standard model of television monitor.

When the speed of the tape is changed on playback, for instance for slowmotion forward or reverse, or fast motion, the head deviates from thecentral path 36. For example, if the tape is brought to a full stop, thehead will trace a path such as that designated by the dotted line 39.For speeds between a full stop and normal speed, the head will trace apath lying somewhere between the dotted lines 36 and 39. When the speedis doubled from normal speed, path 40 is followed, the path fallingbetween dotted lines 36 and 40 for fast speeds between normal and twicenormal speed. When the direction of the tape is reversed for reverseslow motion, the path angles off slightly more than path 39.

As the path of the head deviates from the normal, centered path, itcrosses over the guard band 35 between tracks 34 and produces theundesired noise band in the video picture viewed on the televisionmonitor. During forward slow motion and for successive passes of thehead across the tape, the crossover point on the successive guard bands35 progresses from the bottom of the tape to the top.

In the present invention, the fields are recorded on the magnetic tape32 as shown in FIG. 4 wherein two fields 41 and 42 with a separatingvertical sync pulse 37' are recorded in each track across the tape. Thevideo camera thus produces 120 fields per second rather than thestandard 60. It should be mentioned that FIGS. 3 and 4 are not drawn toscale, e.g. the vertical sync pulses are grossly enlarged relative tothe track length.

The horizontal sync pulses 38 are recorded so that they fall intoalignment across the video tracks. This is to insure that the videohead, when crossing or bridging from one video track to another, willencounter the horizontal sync pulses on the two adjacent video tracks atthe same point in time, to thereby provide horizontal stability to thepicture.

It can be seen that, if the lower fields 41 are termed the even fieldsand the upper fields 42 are termed the odd fields, the crossover pointsbetween tracks will fall either in the odd fields or the even fields butnot in both at any one time, except where the crossover occurs in themiddle or at the edges of the tape.

Referring now to FIG. 5, there is shown one preferred embodiment of thecontrol circuitry utilized in the present invention. In order tounderstand the operations of this schematic diagram, it will be assumedthat the video recorder 10 is operating in the playback mode in slowforward motion and that the noise produced by the head crossing over theguard band between video tracks is occurring during reproduction of theeven pictures or fields so that the even pictures are being blanked outand the odd pictures are being displayed on the monitor.

The circuit of FIG. 5 is in the condition shown in the left-hand end ofthe traces of FIG. 6. The lower output of the master flip-flop circuit84 (trace 86) is low and the upper output is high (trace 85), the outputof the flicker control pulse one-shot circuit 129 (trace 131) to thetelevision monitor is high, and the output of the memory flip-flop 113(trace 114) is high. The pulses of noise due to the head crossover areoccurring at periodic intervals on the lead 110 from the video recorder10 (trace 111), the number of noise pulses per unit of time beingdetermined by the tape speed.

It should be noted that no noise pulses appear during certainrevolutions of the magnetic head since, during one or more revolutions,the head will move across a video track but does not leave the track tocross over to the adjacent track. The exact number of noise pulses andtheir spacing is a function of the tape drum, recorder head and tapegeometry, the track layout on the tape including video track and guardband width, and the speed of the tape adopted during playback.

A tachometer apparatus for producing 120 pulses per second comprises alight reflector strip 43 secured to the head disk 27 of the video taperecorder 10 which also carries the magnetic head 31 in the peripherythereof. The reflector strip 43 passes under the two light beamsproduced by the two light sources 44 and 45 which are spaced 180° aparton the path of rotation of the head disk. Two light detectors 46 and 47are positioned to intercept the light reflected by strip 43 from the twolight sources 44 and 45, respectively, each detector detecting a pulseof light once for each revolution of the head.

The tachometer pulses from the light detector 46, the even field pulses,are transmitted to a first one-shot circuit 48, while the pulses oflight from the second light detector 47, the odd field pulses, aretransmitted to a second one-shot circuit 49. The trailing edge of theoutput pulses from the two tach pulse delay one-shot circuits 48 and 49serve to trigger pulse outputs from the two associated cursor pulseone-shot circuits 51 and 52, respectively. One shots 48 and 49 areprovided with variable resistors 53 and 54 for controlling the timeduration of their output pulses and thus the start time of the outputpulses of one-shots 51 and 52. The 1.5 millisec. output pulses ofone-shots 51 and 52 may, therefore, be adjusted in time so as to beexactly 180° apart, to compensate for any slight deviation from thisspaced relationship by the tachometer pulses.

During the normal speed mode of operation of the video recorder, thevertical sync signals recorded on the video tape are utilized by therecorder as the vertical sync signals for the television monitor.However, if desired, the tachometer pulses could be utilized to developthe vertical sync pulses even during the normal motion of operation andthe vertical sync pulses could be eliminated from the magnetic tape.When the speed of the video recorder is changed from the normal speed inplayback, the vertical sync signals reproduced from the video tapeappear mutilated and are not usable since they will result in a verticaljitter in the picture on the monitor. Therefore, new vertical syncsignals are created from the output pulses of the one-shots 51 and 52for utilization by the monitor in lieu of the vertical sync signals fromthe tape. The negative going output pulses are transmitted to a NANDgate 55 which serves as a cursor pulse combiner, the output pulseserving to trigger a sync pulse delay one-shot circuit 56. The outputpulses from the NAND gate 55 are shown in trace 57 in FIG. 6, the outputpulses corresponding to the output pulses from circuit 51 being labeled(2), or even, and those from circuit 52 being labeled (1), or odd. Thetime duration of the one-shot 56 is controlled by the analog voltageoutput of a ramp generator 58 coupled over lead 59 on through theoperational amplifier 60 to the resistor 61 in the one-shot 56. Anincreasing voltage output from the ramp generator 58 and amplifier 60(trace 62) will produce a decreasing time duration for the output pulseof one-shot 56 (trace 63). The trailing edge of the output pulse ofone-shot 56 triggers a sync pulse generator one-shot circuit 64 toproduce the vertical sync pulses (trace 65) transmitted to the videorecorder monitor circuit over lead 67 for use in lieu of the sync pulseson the video tape.

The variable width sync delay pulses (trace 63) compensate for the factthat, as the magnetic head deviates from the center of the video tracks,it encounters the beginning of a field at a slightly different time inthe head cycle than the beginning of the field was encountered in thepreceding cycle. This can be more easily understood by referring to FIG.7 which shows the beginning end of a recorded video track 34 includingthe vertical sync region 37. Three successive paths for the magnetichead in slot motion are shown as dotted lines (a), (b) and (c). It canbe seen that the time for the head to traverse path (a) from the bottomedge of the tape to the end of the vertical sync region is longer thanthe time for the progressively shorter paths (b) and (c). This change intiming for successive cycles, if not corrected, results in a verticaljitter of the displayed picture on the monitor. By utilizing the linearramp to produce progressive changes in the width of the successive syncdelay pulses (trace 63), this timing problem is corrected and thevertical jitter eliminated.

The detailed operation of the ramp generator 58 will be given below.

The sync pulses are also transmitted to a blanking generator one-shotcircuit 66 which produces an output pulse (trace 66') for each syncpulse, the leading edge of the blanking pulse being in synchronism withthe leading edge of the sync pulse.

The sync pulses are also transmitted over lead 67 to one of the inputsto each of two NAND gates 68 and 69, which also have input leads coupledto the positive going pulse outputs of the one-shot circuits 51 and 52,respectively. The time duration of the alternate pulses from one shots51 and 52 are made long enough to span the time of the corrected syncpulses received from the sync generator 64. The output pulses from theNAND gate 68 are coupled to one input of a square wave output flip-flopcircuit 71 while the output pulses from the NAND gate 69, whichalternate in time with the pulses from NAND gate 68, are coupled to theother input to the flip-flop 71. The square wave output from flip-flop71 on lead 72 is shown in trace 73 and the inverted square wave outputon lead 74 is shown in trace 75. It can be seen that the leading edgesof each square wave are in synchronism with the corrected sync pulses,trace 65.

The output lead 72 from flip-flop 71 is connected to one of the inputleads to NAND gate 76 while the other output lead 74 is coupled to oneof the input leads to NAND gate 77. The other two input leads to the twoNAND gates 76 and 77 are coupled to the output of the blanking pulsegenerator 66. The negative going blanking pulses (trace 66') delay theoutputs of NAND gates 76 and 77 from going negative when theirrespective inputs from the flip-flop 71 go positive until the negativegoing blanking pulse terminates. This results in square wave outputsfrom NAND gate 76 (trace 79) and NAND gate 77 (trace 81) which have alonger positive going half cycle than negative going half cycle by twicethe length of the blanking pulse.

The output of NAND gate 76 is coupled to one of the inputs of each oftwo exclusive OR gates 82 and 83 while the output of NAND gate 77 iscoupled to a different one of the inputs to the OR gates 82 and 83.

During this period of operation when the lower output of masterflip-flop 84 is low (trace 86) and the upper output is high (trace 85),the output of the exclusive OR gate 83 (trace 83') will go low when theoutput on NAND gate 77 (trace 81) goes high. The output of gate 83 willremain low until the output of NAND gate 77 (trace 81) goes low.

Stop motion can be enabled in the recorder circuit by a switch whichstops the recorder tape drive; operation of this circuit when the stopaction switch is operated is described below. Until the stop actionswitch is operated, negative voltage is standing on lead 86. One inputto NAND gate 87 is thus low, and one input to NAND gate 88 from theinverter 89 is high. The positive going sync pulses from the syncgenerator 64 on the other two inputs to the two NAND gates 87 and 89result in the flip-flop 91 being maintained in one state with its upperoutput lead high and its lower output lead low. Thus the input lead 92to exclusive OR gate 93 is maintained high and the input lead 94maintained low. At the time the output of OR gate 83 goes low (trace83'), a high exists on the output of gate 82. The inverters 95 and 96invert these two outputs so that the input lead 97 to OR gate 93 is highand the input lead 98 goes high to cause the output of the OR gate 93 togo low. Thus, as gate 83 goes low, the output lead 99 of OR gate 93 tothe television monitor goes low to turn the picture off at the monitor.This lead 99 remains low until the output of gate 83 again goes high, atwhich time this lead 99 to the monitor goes high to turn the monitorpicture ON.

The gate 83 is low during the reproduction time of the even pictures,those on the lower half of the magnetic tape, including the blankingpulse periods at the beginning and end of the even pictures, and thusthe even pictures are blanked out on the monitor screen. The output ofgate 83 is high during the reproduction time of the odd pictures, theupper half of the video tape, and thus the odd pictures are displayed onthe monitor. Since the crossover noise pulses are being produced duringthe time of the even pictures and these even pictures are being blankedout, the undesirable noise streaks are not seen on the monitor.

When the two upper input leads to the exclusive OR gate 82 assume acommon state relative to the other common state of the two lower inputleads, the output of OR gate 82 will assume the one or high state. Thisoccurs, for example, when the output of the NAND gate 76 goes low (trace79) while the output on the lower lead of master flip-flop 84 is low(trace 86) and the outputs of NAND gate 77 and the upper lead of masterflip-flop 84 are high (traces 81 and 85, respectively).

The high state on the output of gate 82 initiates operation of thesample delay one-shot circuit 101 which produces a negative going pulseoutput (trace 102). This sample pulse is of variable time durationdetermined by the voltage on the gate of the FET 103 for reasonsdescribed below. At the end of the time delay of the one-shot circuit101, the output goes high to the associated input to NAND gate 104. Asecond input to gate 104 is high from the output of gate 82. The thirdinput from inverter 105 is normally high, and, therefore, the output ofNAND gate 104 to one of the inputs of NAND gate 106 goes low. One inputto NAND gate 107 is normally low so that the output of gate 107 to theother one of the inputs to NAND gate 106 is normally high. Therefore,the output of NAND gate 106 goes high (trace 108) at one of the inputsto the NAND gate 109.

NAND gates 105 and 107 come into play in the reverse mode of operationof this circuit which will be described in detail below.

Since the video recorder is operating at a slow motion speed, noisepulses (trace 111) due to the head crossing over the guard band arebeing generated in the recorder circuitry, and these noise pulses aretransmitted over lead 110 and through amplifier circuit 112 and appearas positive going pulses at one of the input leads to NAND gate 109.Those noise pulses occurring during the early portion of a field trackon the tape will appear at gate 109 early in the time cycle and beforethe sample pulse (trace 108) appears on the other input lead and theseearly noise pulses will result in no output from the gate 109.

If there is no coincidence between the noise pulse (trace 111) and thesample gate pulse (trace 108), the NAND gate 109 will remain unchangedand the output of gate 106 will go low when the output of gate 82 goeslow due to the output of NAND gate 76 (trace 79) going positive.

As the noise pulses begin to make their appearance closer to the end ofthe field which is being blanked out, one of these noise pulses, forexample, the one which is second from the last noise pulse at the end ofthe field, coincides with the input pulse (trace 108) on gate 109 and anegative going pulse (trace 112) appears on the input to the memoryflip-flop 113. The output of flip-flop 113 goes low (trace 114) untilthe receipt of the next negative going blanking pulse (trace 66') fromthe blanking generator 66. The flip-flop 113 operates on the leadingedge of the blanking pulse and triggers the delay one-shot 115 whichoperates to produce a 22-millisecond pulse (trace 116). The positivegoing trailing edge of this delay pulse triggers a 6-millisec. delayone-shot 117 which operates to produce a 6-millisec. output pulse (trace118).

The zero state output of the one shot 115 is coupled to the input ofNAND gate 109 to disable this gate so that subsequent noise pulses willnot be allowed to operate the flip-flop 113 during the 22-millisec.delay time.

It can be seen from traces 116 and 118 that the time duration ofone-shot 115 is slightly less than one and one-half revolutions of therecorder head and that the time duration of one-shot 117 spans over theperiod of the third and fourth half revolutions after initial operationof one-shot 115.

The positive going output of one-shot 117 appears on the inputs to twoNAND gates 119 and 121, the other inputs of NAND gates 119 and 121 beingconnected to the outputs of NAND gates 68 and 69 through inverters 122and 123, respectively. On initiation of the next sync pulse from syncgenerator 64 to gates 68 and 69 during the ON time of the pulse fromone-shot circuit 51, the output of NAND gate 119 goes positive (trace121) and triggers the master flip-flop 84 to cause its upper output lead(traces 85) to go to the zero state and its lower output lead (trace 86)to go to the one state.

Thereafter, the output of exclusive OR gate 83 (trace 83') will go highwhen the output on NAND gate 76 (trace 79) goes low and low when gate 76goes high. Gate 83 will go low during the reproduction time of the oddpictures, those on the upper half of the magnetic tape, including theblanking pulse periods at the beginning and end of the odd pictures.Therefore, the negative going output pulses on the output of gate 83will cause the odd pictures to be blanked from the monitor and the evenpictures to be reproduced. Since the noise pulses will now be occurringduring the reproduction time of the odd pictures, the crossover noisewill be blanked out. Only the noise-free even pictures will bereproduced until such time as the noise pulse (trace 111) againcoincides with the sample time pulse (trace 108) to produce thetriggering pulse (trace 112) for the memory flip-flop circuit 113, and,ultimately, the switching of master flip-flop 84.

Before the switching of the master flip-flop 84, the odd pictures areappearing on the monitor alternating with blank periods during the evenpicture period. Because of the speed at which this picture on-off cycleis taking place, the human eye does not detect the blank periods on themonitor. However, at the switchover from reproduction of the oddpictures on the monitor to reproduction of the even pictures or fields,the last odd picture shown is followed immediately by the first evenpicture without the ordinary blank period between pictures. This can beseen on trace 83' which shows two adjacent positive pulses at the timeperiod when the master flip-flop switches (traces 85 and 86). These twosuccessive ON times will appear to the human eye as a sudden, momentarybrightening of the picture on the monitor. The succession of periodicpicture brightenings will take the form of a flicker, which isdisturbing to the viewer. In order to eliminate this flicker, thepresent circuit is provided with a means to dim the last picture and thefirst picture at each changeover point to thus keep the brightness ofthe picture reproduction over these two particular fields at a constantlevel to the human eye.

When the output of memory flip-flop 113 goes high (trace 114), ittriggers operation of a delay one-shot circuit 124 to produce a14-millisec. negative going output pulse (trace 125). At the terminationof this pulse, delay one-shot 126 is triggered to produce a 6-millisec.positive going pulse (trace 127) on one input to the NAND gate 128.

The timing of one-shots 124 and 126 is selected so that the ON time ofone-shot 126 (trace 127) bridges the start time of the last odd pictureto be displayed before switching to display of the first even picture,and when the vertical sync pulse (trace 65) from sync pulse generator 64is received on the other input of NAND gate 128 at the start of the lastodd picture, the NAND gate produces an OFF state pulse at its output.This negative going pulse triggers operation of the two-field one-shotcircuit 129 to produce an output pulse (trace 131) to the brightnesscontrol circuitry in the monitor control to dim the picture. Theoperation time of one-shot circuit 129 is set to span two pictures orfields; the end of the pulse (trace 131) extends into the time period ofthe blank picture following the first even picture and its terminationtime is therefore not critical.

As the speed of the video tape relative to the speed of the recorderhead changes from the normal speed, the number of noise pulses per unitlength of time changes. For example, as the tape decreases in speed, andthe picture motion becomes slower, the number of noise pulses that occurover the length of the cyclic reproduction of one field region (i.e. oddor even field region) increase in number. The present system is arrangedto sample the last three noise pulses in the one-half of the video trackbeing blanked before switching over to blank the fields from the otherhalf of the video track. Because of the variable spacing between theselast three pulses with varying slow motion speeds, the time duration ofthe sample gate one-shot 101 is made variable with tape speed.

When the recorder is switched from normal speed to the "slow motionmode", the recorder tape drive is switched from an A.C. driving motor toa D.C. motor, the speed of the tape being controlled by the D.C. voltageapplied to the D.C. drive motor, the lower the voltage, the slower thespeed. Assume that, in switching from normal speed to slow motion, thespeed is switched to 25 percent of normal and a certain level of D.C.voltage appears on lead 132. This voltage controls the voltage levelapplied to the base of emitter follower transistor 133 to control thevoltage applied to the FET 103 and thus control the time length of thesample delay pulse (trace 102). At a slow motion of 25 percent of normalmotion, the delay pulse (trace 102) is made just long enough to give asample gate pulse (trace 108) wide enough to bridge the last three noisepulses at the end of a field region. As the motion becomes slower andthe rate at which the noise pulses occur increases, the D.C. voltage onlead 132 becomes lower and produces an increase in the length of thesample delay pulses (trace 102). The time of the sample gate (trace 108)is thus shortened so as to bridge only the more rapidly occurring lastthree noise pulses.

At the time the output of delay one-shot 117 to one input of NAND gate134 is high, the next positive going pulse from the synce pulse delayone-shot 56, synchronous with the negative going pulse shown in trace63, appearing at the other input to NAND gate 134 causes the output ofgate 134 to go high. The output of inverter 135 then goes low andtriggers the one-shot circuit 136. The positive pulse output of one-shot136 pulls the ramp generator 58 down and starts the ramp generator on anew charge-up cycle.

The ramp generator includes a pulse input wave shaping circuit formed byresistor 137 and condenser 138. Resistors 139 and 141 form a part of theoutput amplifier circuit and capacitor 142 serves as a by-passcondensers. Resistors 143 and 144 control the base or starting voltagelevel for the ramp while resistors 145 and 146 control the peak voltagelevel, the difference between these two voltage levels controlling thewidth of the sync delay pulse output of one-shot 56. Capacitor 147 isthe main charging capacitor of the ramp.

FET 148, resistors 149 and 151, and the resistance of photoconductor152, form a controllable constant current source, the currentdetermining the slope of the ramp output.

As the motion decreases from normal motion to slower motion, the fieldscan rate, i.e. the number of different fields scanned per unit time,decreases, the noise pulse rate increases, and the time period betweenfield or picture switchover times increases. Since the ramp output mustrise from the base voltage level to the peak voltage level during theperiod between successive switchover times, the slope of the ramp mustdecrease as the switchover times increase, i.e. as the motion becomesslower.

As mentioned above, the D.C. voltage on lead 132 decreases as thepicture motion becomes slower, the this D.C. voltage is used to controla lamp 153 associated with the photoconductor 152. The lower the D.C.voltage, the darker the lamp and the higher the resistance of thephotoconductor 152. An increase in the photoconductor resistanceproduces a decrease in the current flow through the FED circuit to thecapacitor 147, and thus a decrease in the ramp slope. Conversely, anincreasing D.C. voltage with increasing picture motion gives anincreasing current flow to capacitor 147 and an increased ramp slope.The low voltage appearing on the output of inverter 89 during the stopmotion mode of operation of the recorder, described more fully below,operates transistor 155 to clamp the output of the ramp generator to aselected voltage level between the base and peak levels determined bythe setting of the resistance network including resistors 156, 157 and158.

When the recorder is switched to the "reverse" mode of operation and runat slow motion, the direction of movement of the tape is reversed andthe crossover noise points occur at the top of the tape and progressdown, the noise band appearing at the bottom of the picture on themonitor and moving up the picture. It is therefore necessary to positionthe sample gate pulse (trace 108) at the beginning end of a field ratherthan the finishing end. When the recorder is placed in the reverse mode,positive voltage is connected to lead 161. The output of inverter 105goes low and serves to decouple the sample delay one shot 101 from theNAND gate 109. The output of NAND gate 104 goes high. The output of theexclusive OR gate 82 now serves to trigger the one-shot 162, when gate82 goes high at the beginning of a field, the output of one-shot 162goes high to NAND gate 107. The two highs on the input to gate 107produce a low on its output to NAND gate 106 which causes the output ofgate 106 to go high to NAND gate 109. A sample pulse similar to trace108 is therefore created at the beginning of each field and awaitscoincidence with a noise pulse (trace 111) to trigger the memoryflip-flop 113 as described above.

The length of this sample pulse output of NAND gate 106 is controlled bythe timing circuit of the one-shot 162 including the FET circuit 163 andthe inverting network including transistor 164 and associated circuitry.As mentioned above, the tape speed is decreased by decreasing the D.C.voltage applied to the D.C. tape drive motor in the recorder. Thisdecreasing voltage appears on lead 132 to the voltage inverting networkof transistor 164, and an increasing output is transmitted to the timingcircuit of one-shot 162. An increasing voltage produces a shorter pulsetime for one-shot 162 and thus a shorter sample time to bridge only thelast three noise pulses occurring at the beginning end of the fields.

In reverse, the high on lead 161 is also used to invert the ramp outputsignal from the operational amplifier 60 to reverse the time delays ofthe successive sync delay pulses (trace 63).

When the stop motion switch of the recorder is operated, negativevoltage is removed from lead 86 and the output of inverter 89 goes low,transistor 154 is turned on and the voltage on the lead to the timingcircuit including FET 103 of sample delay one-shot 101 goes high. Theoperate time of one shot 101 becomes very short (trace 102) and thesample time of NAND gate 109 (trace 108) becomes the total time of acomplete field. The sample gate is thus opened wide so that the nextincoming noise pulse, wherever it occurs in either of the two fieldregions, will cause the memory flip-flop 113 to operate and start theswitchover cycle. Since the switchover occurs at the end of the thirdfield region, i.e. one and one-half head revolutions after receipt ofthe noise pulse, the field region selected for viewing will be the onein which the crossover noise is occurring. This system is designed toautomatically switch to the opposite, noise-free field region.

When the negative potential is removed from lead 86 and the input leadto NAND gate 87 goes high and the input lead to NAND gate 88 goes low,the flip-flop circuit 91 operates to make its upper output lead 92 lowand its lower output lead 94 high. The exclusive OR circuit 93 operatesto reverse its output on lead 99 to blank out the picture that themonitor would normally be showing in stop motion and display theopposite picture.

The lower three traces in FIG. 6 illustrate (a) the periods of rotationof the magnetic head, (b) the halves of the tape being selected fordisplay on the monitor, and (c) the frames or pictures selected andnumbered consecutively.

It will be noted that in this system as described the pictures taken bythe video camera and recorded on the video tape are at the rate of 120per second. Thus pictures are reproduced at the rate of 60 per second,and this two-to-one speed ratio enhances the slow motion characteristicsof the displayed picture. To further enhance these slow motioncharacteristics, the number of fields recorded in each track can beincreased, e.g. the camera can take 180 or 240 pictures per second andrecord three or four fields per track. With the crossover noiseappearing in one of the field regions, the first or second adjacentfield region is selected for reproduction, and the picture on themonitor blanked out during the other two or three field region periods,including the one containing the noise. Since the pictures taken at 180or 240 per second are being displayed at the rate of 60 per second, theslow motion aspects of the displayed picture are substantially enhanced.The fact that the picture is on for only one-third or one-quarter of thetime does not noticeably detract from the picture quality, and theseduty cycles are clearly acceptable. Thus, the number of fields recordedper unit time can be increased as desired within acceptable lowpercentage duty cycles.

The present invention, although described with reference to its use witha 360° wrap helical scan recorder, is equally applicable for use withthe omega wrap helical scan recorders using two, 180° spaced recordheads. The invention may also be used with the spiral recorded discvideo recorders as well as spiral drum recorders.

Referring now to FIG. 8 there is shown in block diagram form anotherembodiment of the present invention in which the duty cycle of thetelevision monitor is increased to 100 percent. This embodiment utilizesa major portion of the apparatus shown in FIGS. 5A through 5D and whichis not shown again in FIG. 8. The flicker correcting circuitrycomprising elements 124, 126, 128, and 129 is not utilized. Thenoise-free half of the video tape is reproduced twice during each cycle,the second reproduced field being used to fill in the blank frame periodprovided in the above described embodiment.

The head disk 27 is provided with a second magnetic head 171 spaced 180°from the first head 31, this second head being coupled through a secondvideo amplifier circuit 172 to a video switch 173. The output of thevideo amplifier in the video recorder system 10 associated with thefirst magnetic head 31 is coupled to an associated video switch 174.

Operation of the two video switches is controlled from the logiccircuitry of FIGS. 5A through 5D. The output of the exclusive OR gate 93operates video switch 174 to couple the output of the first videoamplifier 10 through to the video combiner circuit 175 which transmitsthe video signal to the television monitor during the ON time ofexclusive OR gate 93 (trace 83'). Thus the noise-free field from head 31is reproduced on the monitor.

The output of OR gate 93 is also coupled to one input of NAND gate 176,the other input to NAND gate 176 being coupled to the output of blankinggenerator 66. During the OFF time of OR gate 93 and during the periodthat no negative going pulses are appearing from the blanking generator66 at the beginning and the end of OFF time of OR gate 93, the output ofNAND gate 176 goes high to turn on the video switch 173 and couple thesecond video amplifier 172 through the video combiner 175 and on to thetelevision monitor. The noise-free field region being reproduced fromthe head 171 is thus displayed on the monitor during the time the firsthead is encountering the crossover noise. The video switches 173 and 174thus operate to alternately switch between the two heads 31 and 171 todisplay successive noise-free fields without the intervening blank fieldperiod.

I claim:
 1. The method of recording video signals on a magnetic tape ofa video tape recorder at a normal longitudinal tape speed andreproducing said video signals from said magnetic tape at normallongitudinal tape speeds and at speeds different from normallongitudinal tape speeds comprisng the steps ofrecording a plurality ofseparate non-redundant video fields in series in each one of a sequenceof video tracks recording along the tape, scanning the video tracks witha magnetic reproduce head to reproduce .[.the recorded fields inorder.]. .Iadd.at least one recorded field per track .Iaddend.fordisplay on a video display means, said reproduce head crossing over thespace on the magnetic tape between adjacent video tracks and producing anoise on the video signal when the longitudinal tape speed is changedfrom the normal speed, and .Iadd.detecting the occurrence of said noiseand .Iaddend.blanking out that particular .Iadd.noisy .Iaddend.field ineach track at which the head crossover between adjacent tracksoccurs.Iadd., whereby at least one noise-free field in each track isreproduced for display.Iaddend..
 2. The method as claimed in claim 1including the step of reproducing for display on the video display meansanother field on said track during the period of time said particularfield is being blanked out.
 3. The method of recording video signals ona magnetic tape of a video recorder at a normal longitudinal tape speedand reproducing said video signals from said magnetic tape at normallongitudinal tape speeds and at speeds different from normallongitudinal tape speeds comprising the steps ofrecording a plurality ofseparate non-redundant video fields in series in each one of a sequenceof video tracks recorded along the tape, scanning the video tracks witha magnetic reproduce head to reproduce .[.the recorded fields inorder.]. .Iadd.at least one recorded field per track .Iaddend.fordisplay on a video display means, said reproduce head crossing over thespace on the magnetic tape between adjacent video tracks and producing anoise on the video signal when the longitudinal tape speed is changedfrom the normal speed, detecting the occurrence of said noise atcrossover, and blanking out from display in response to the detection ofsaid noise that particular field in each track at which the headcrossover between adjacent tracks occurs.Iadd., whereby at least onenoise-free field in each track is reproduced for display.Iaddend.. 4.The method as claimed in claim 3 including the step of reproducing fordisplay on the video display means another field on said track duringthe period of time said particular field is being blanked out.
 5. Themethod as claimed in claim 3 including the step of producing a correctedvertical sync signal for each of the vertical sync periods between thefields recorded on the magnetic tape for use by the signal displaymeans.
 6. The method as claimed in claim 5 including the step ofproducing tachometer pulses from the rotating head assembly of the videorecorder, said tachometer pulses serving to produce said correctedvertical sync pulses, and producing a linearly sloping analog signal foradjusting the time spacing between said corrected vertical sync pulses.7. A video recorder system for reproducing a plurality of separatenon-redundant video fields recorded in series in a track across amagnetic tape, the tracks being recorded sequentially along the tape,comprisingreproducing means including a reproduce head for scanningalong each series of fields in each track in sequence to reproduce.[.the separate fields in order.]. .Iadd.at least one recorded field pertrack .Iaddend.for display on a signal display means, means for changingthe .[.relative speed of the magnetic tape and the reproduce head.]..Iadd.longitudinal tape speed from the longitudinal tape speed duringrecording .Iaddend.to change the speed at which the video tracks arereproduced, whereby the reproduce head crosses over the space on themagnetic tape between adjacent tracks, .[.and.]. .Iadd.means fordetecting the occurrence of noise generated at the head crossover and.Iaddend.means for blanking out that particular .Iadd.noisy .Iaddend.oneof the plurality of fields in each track at which said crossoveroccurs.Iadd., whereby at least one noise-free field in each track isreproduced for display.Iaddend..
 8. A video recorder as claimed in claim7 including means for reproducing another field from said track fordisplay on said signal display means during the period of time saidparticular field is being blanked out.
 9. A video recorder as claimed in8 wherein said means for reproducing said other field comprises a secondreproduce head spaced from the first reproduce head, and means forswitching the display means from said first reproduce head to saidsecond reproduce head during the period of time said first head isscanning the field at which said crossover occurs.
 10. A video recorderas claimed in claim 7 including means for producing a corrected verticalsync signal for each of the vertical sync periods between the fieldsrecorded on the magnetic tape for use by the signal display means.
 11. Arecorder as claimed in claim 10 including means for dimming thebrightness of said signal display means during the reproduction times ofthe field displayed immediately before and the field displayedimmediately after the switchover from blanking said particular one fieldto blanking said different field.
 12. A video recorder system forreproducing a plurality of separate non-redundant video fields recordedin series in a track across a magnetic tape, the tracks being recordedsequentially along the tape, comprisingreproducing means including areproduce head for scanning along each series of fields in each track insequence to reproduce .[.the separate fields in order.]. .Iadd.at leastone recorded field per track .Iaddend.for display on a signal displaymeans, means for changing the .[.relative speeds of the magnetic tapeand the reproduce head.]. .Iadd.longitudinal tape speed from thelongitudinal tape speed during recording .Iaddend.to change the speed atwhich the video fields are reproduced, whereby the reproduce headcrosses over the space on the magnetic tape between adjacent tracks andproduces a burst of noise at the crossover, and means for detecting saidnoise and operating in response thereto for blanking out that particularone of the plurality of fields in each track during which said crossoveroccurs.Iadd., whereby at least one noise-free field in each track isreproduced for display.Iaddend..
 13. A video recorder as claimed inclaim 12 including means for reproducing another field from said trackfor display on said signal display means during the period of time saidparticular field is being blanked out.
 14. A video recorder as claimedin 13 wherein said means for reproducing said other field comprises asecond reproduce head spaced from the first reproduce head, and meansfor switching the display means from said first reproduce head to saidsecond reproduce head during the period of time said first head isscanning the field at which said crossover occurs.
 15. A video recorderas claimed in claim 12 including means for producing a correctedvertical sync signal for each of the vertical sync periods between thefields recorded on the magnetic tape for use by the signal displaymeans.
 16. A recorder as claimed in claim 15 including means for dimmingthe brightness of said signal display means during the reproductiontimes of the field displayed immediately before and the field displayedimmediately after the switchover from blanking said particular one fieldto blanking said different field.
 17. In a system for reproducing videosignals from a magnetic tape recorded at a predetermined longitudinaltape speed and having a plurality of video tracks angularly disposedacross said magnetic tape and spaced by guard bands, each of said videotracks including a plurality of complete, successive, non-redundantfields, said fields positioned in said tracks so that the portions ofsaid video tracks at each tape edge contain at least a segment of avertical blanking interval, the combination comprisingmeans for scanningsaid video tracks to reproduce .[.said fields.]. .Iadd.at least onefield per track.Iaddend., means for longitudinally transporting saidmagnetic tape at said recorded tape speed or at another speed, and meansfor .[.blanking out reproduced fields containing.]. .Iadd.detecting.Iaddend.noise picked up by said scanning means from said guard bandswhen said magnetic tape is transported at speeds different from saidrecorded tape speed.Iadd., and means for blanking out reproduced fieldscontaining said noise, whereby at least one noise-free field in eachtrack is reproduced.Iaddend..
 18. The combination of claim 17 whereinsaid other speed is zero whereby a still frame is reproduced.
 19. Thecombination of claim 17 wherein said other speed has a direction of tapetravel reversed from the direction of tape travel during recordingwhereby reverse action motion is reproduced.
 20. The combination ofclaim 17 further comprisingmeans for producing corrected vertical syncsignals.
 21. The combination of claim 20 further comprisingmeans fordisplaying said reproduced fields, and means for dimming the brightnessof said signal display means during the reproduction times ofconsecutively displayed adjacent fields.
 22. In a system for recordingand reproducing a video signal on magnetic tape, the combinationcomprisinginput means for providing a video signal having a field rateof n times 60, where n is a whole positive integer, tape transport meansfor longitudinally transporting said tape a first predetermined speedduring recording and for longitudinally transporting said tape duringreproducing at said first predetermined speed or at another speed,recording means receiving said video signal for recording said videosignal on said magnetic tape in a plurality of video tracks angularlydisposed across said magnetic tape and spaced by guard bands, each ofsaid video tracks including a plurality of complete, successive,non-redundant fields, said fields positioned in said tracks so that theportions of said video tracks at each tape edge contain at least aportion of a vertical blanking interval, means for scanning said videotracks to reproduce .[.said fields.]. .Iadd.at least one field pertrack.Iaddend., means for .[.blanking out reproduced field containing.]..Iadd.detecting .Iaddend. noise picked up by said scanning means whensaid magnetic tape is longitudinally transported at speeds differentfrom said first predetermined speed.Iadd., and means for blanking outreproduced fields containing said noise, whereby at least one noise-freefield per track is reproduced.Iaddend..
 23. The combination of claim 22wherein said other speed is zero whereby a still frame is reproduced.24. The combination of claim 22 wherein said other speed has a directionof tape travel reversed from the direction of tape travel duringrecording whereby reverse action motion is reproduced.
 25. Thecombination of claim 22 wherein said recording means provides that thehorizontal blanking intervals in each video track are substantiallyaligned with the horizontal blanking intervals in adjacent video tracks.26. The combination of claim 25 wherein n is a whole positive integergreater than one.
 27. The combination of claim 26 further comprisingmeans for producing corrected vertical sync signals.
 28. The combinationof claim 27 further comprising means for displaying said reproducedfields, andmeans for dimming the brightness of said signal display meansduring the reproduction times of consecutively displayed adjacentfields.